The invention relates to a cordless telephone unit for making and receiving calls over a telephone network; and more particularly to an improvement in such units which minimizes the cost and effort of verifying that the handset is within range of a fixed station.
The cord between a telephone set and the telephone network connection is generally several meters long. The cord between the handset or receiver and the telephone set, such a desk set or wall-mounted set, is still usually considerably shorter. Consequently, a user is bound to a certain spot while initiating a call, and can move only a short distance from that spot during the conversation.
Greater freedom is provided by so-called handsfree facilities, such as Speakerphone set having a loudspeaker and a sensitive microphone, enabling the user to move about freely for several meters in the room during a telephone conversation. However, it is still necessary to touch the fixed unit in order to initiate or receive a call.
To increase the user's range even more, cordless telephones are increasingly used in various countries. With these telephones, the transmission of information between a fixed station and a portable handset, together forming a cordless telephone unit, is effected through a pair of radio transmission channels, forming a duplex pair. The fixed station is connected via connector box to a main station or a private branch exchange, to provide access to the public telephone network. Since the fixed station and handset are connected to each other over radio transmission channels, the range of free movement is increased to as much as 200 meters or more from the associated fixed station or connector box.
In the Federal Republic of Germany there are 40 channels available for transmission of information, lying the 900 MHz. frequency range. In this frequency range, and with the limited transmission distance, the same radio channel is not apt to be seized by mutuallly adjacent cordless telephones. If a user wishes to initiate a call, and operates the "cradle switch" of the handset, or the fixed station receives a ringing signal over the network connection, then a station searcher which is part of the handset receiver, or part of the fixed station, will commence search for an idle radio channel in the assigned band, and upon finding such a channel will seize the duplex pair. To minimize the possibility that messages transmitted between the fixed station and handset of one cordless telephone can be overheard by another cordless telephone, to permit correct allocation of charges, and to ensure that the allocation of pairs of channels is unambiguous, an identification number is assigned to each cordless telephone independent of the subscriber's telephone number.
A method of searching for a radio channel and exchanging identification numbers, when a radio connection between handset and fixed station is set up, is further described and explained in European Patent Specification EP-B No. 10 074 940. According to this method, an idle transmission channel is seized by energizing the transmitter of the set from which communication is to be established, and a data packet containing the identification number is transmitted over the channel. This set then waits for an acknowledgement data packet, containing the same identification number to be transmitted by the corresponding opposite set. Upon receiving the identification number, on acknowledgement message, from the opposite station this number is compared by an arrangement in the initating station to is own identification number. If the two numbers are identical, the connection is switched through to the telephone network by a control arrangement, the exchange of identification numbers is stopped and the existing radio connection is monitored.
An article in ntz, Vol. 38 (1985), No. 7, pages 468-471, describes a cordless telephone system for the 900 MHz. frequency band. The automatic management of the 40 transmission channels is described on page 468. When the fixed station is in a rest position, all 40 channels are searched consecutively, to detect any request from a portable handset to establish a connection to the telephone network. The channels are searched cyclically, to determine if a tested channel is occupied; if it is not occupied, this channel is noted as idle, and the receiver switches to the next channel in the cycle.
Further monitoring, during the course of a conversation, is described on page 470 as an exchange of identification numbers which takes place every 15 seconds. During his exchange of numbers, speech transmission is interrupted for a short time interval, and the transmission of identification number can be heard softly during this interval.
Another method of monitoring a cordless telephone connection is described in Technische Mitteilungen PTT, No. 4, 1986, pages 152-165. In this monitoring method, identification numbers are exchanged to safeguard the assigning of the two stations. Monitoring is further described and explained at page 157: if the channel quality drops below a signal-to-noise ratio of 20 dB, for more than 500 ms., a user is informed by a warning tone emitted by the handset. If the receiving quality does not improve during the next 10 seconds, for example as a result of a change in the user's position, then the set automatically attempts to establish a new radio contact. After a connection has been set up, the identification number as described above is transmitted every 15 seconds and acknowledged automatically by the opposite station. If the acknowledgement signal contains an error twice within 35 seconds, or is not received, a new radio contact is again automatically attempted.
Operation of the systems described above, to establish connections and monitor transmission quality, presupposes that a connection has been made between the subscriber's fixed station and that telephone network. However, these systems do not provide a way for a user to verify whether he is still within the coverage area of the fixed station when the user moves a considerable distance while carrying the handset without utilizing the public telephone network or at least occupying a line selector.